Review of Concentrate Management Options

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					       Review of Concentrate Management Options
                                   Mike Mickley, P.E., Ph.D.1
This paper presents a historical look at concentrate management options used across the U.S. The
paper reviews background statistics and issues that characterize and frame the challenges of
concentrate management. Both National and Texas statistics are provided. Concentrate
management options are discussed in terms of their nature, frequency of use, cost, and future
applicability. Due to the relatively few number of municipal seawater desalination plants in the
U.S. and consequently the limited concentrate disposal experience from such plants, the report
information necessarily reflects and focuses on inland desalination plants and inland disposal of
concentrate.

National Statistics
In the United States membrane processes have been the technology-of-choice to provide new
sources of potable water through treatment of lower quality resources (brackish and saline
waters).
At the start of 2003, 431 municipal membrane plants of size 25,000 gpd or greater had been built
in the 50 states. This included 234 desalination plants (RO, NF, and ED/EDR) and 197 low-
pressure plants (UF and MF) (Mickley, 2004a). Figure 1 illustrates the growing number of plants
with time and Figure 2 shows number of different types of desalination plants that had been
built. Only 4% of the desalination plants are seawater reverse osmosis plants with only the
Tampa Bay plant being larger than 2 MGD in size.


Figures 3 and 4 show the number of plants by state. It can be seen that Texas has the 3rd
highest total number of plants (28, with 20 desalination and 8 low-pressure). It has the 3rd
highest number of desalination plants and 5th highest number of low-pressure plants.




1
    Mickley & Associates


                                               173
Figure 1-Number of desalting plants



                                      200
                                      180
            No. of municipal plants




                                      160
                                      140
                                      120
                                      100
                                       80
                                       60
                                       40
                                       20
                                        0
                                            BRO    NF         EDR       SRO      MF          UF
                                                  Desalting processes         Low -pressure processes

                                                              Membrane Process




Figure 2-Number of municipal membrane plants of different types >0.025 MGD built prior to
         2003 (Mickley, 2004a)




                                                                174
Figure 3-Number of desalting plants by state




Figure 4-Number of low-pressure plants by state


                                               175
The growing size of plants (and thus of concentrate volume) with time may be seen in Figure 5.
Of the desalination plants built prior to 1993, many were smaller than 0.1 MGD and few greater
than 6 MGD. This situation is reversed for plants built between 1993 and 2003.

                         40
                               built
                         35   < 1993

                         30
                         25
            % of total




                                                                              built
                         20                                                1993 - 2003

                         15
                         10               built
                                                                   built
                                       1993 - 2003
                                                                  < 1993
                         5
                         0
                                 < 0.1 MGD                           > 6.0 MGD
                                                     Plant size




Figure 5 Changes in municipal membrane plant size before and after 1993 (Mickley, 2004a)


Nearly all of the 234 desalination plants have used conventional methods of concentrate disposal.
These methods include:
   •   Disposal to surface water
   •   Disposal to sewer
   •   Land application of concentrate
   •   Disposal to evaporation pond
   •   Disposal by deep well injection
Figure 6 shows the use of the different disposal methods. Surface water disposal (106 plants or
45%), disposal to sewer (63, 27%), and disposal via deep well (31, 13%) together account for
85% of the disposal situations.




                                                     176
                                                                 120
                                                                             106



              No. of Municipal Desalting Plants (through 2002)
                                                                 100


                                                                  80

                                                                                          63
                                                                  60


                                                                  40
                                                                                                       31

                                                                                                                     19
                                                                  20
                                                                                                                                   10
                                                                                                                                                   2              2
                                                                      0
                                                                            Surface      Sewer      Deep Well        Land     Evaporation Recycle               Reuse
                                                                             water                   Injection                   pond                           system




Figure 6 Number of desalting water treatment plants > 0.025 MGD by disposal method
         (Mickley, 2004a)


Figure 7 shows the use of the conventional disposal options as a function of plant size. Disposal
to surface waters is used frequently regardless of plant size. The frequency of disposal to sewer
decreases with plant size. The opposite trend is true of disposal to deep well where there are
significant economies of scale. Note that disposal to evaporation pond and land is used only for
small volume concentrates.

                                                                 60


                                                                 50                                          < 1 MGD          1-6 MGD             > 6 MGD
             % of Desalting Plants




                                                                 40


                                                                 30


                                                                 20


                                                                 10


                                                                  0
                                                                          Surface     Sewer    Subsurface   Evaporation     Land        Recycle        Reuse
                                                                           water                Injection      pond                                    system




Figure 7 Percentage of desalting plants >0.025 MGD by disposal method and capacity
         (Mickley, 2004a)


                                                                                                               177
Texas Statistics
The 28 Texas municipal membrane plants operating at the beginning of 2003 include 20
desalination plants (15 BRO and 5 EDR) and 8 low-pressure plants (5 MF and 3 UF). These
plants are listed in Table 1 along with some descriptive information.


Table 1-Texas municipal membrane plants of size > 0.025 MGD built before 2003 (Mickley, 2004a)
                       NAME                             LOCATION           MEMBRANE   PLANT    PLANT     YEAR    SOURCE     FEED         REASON FOR             CONCENTRATE
                                                                             TYPE      TYPE   CAPACITY   START   WATER       TDS         TREATMENT            DISPOSAL METHOD
Haciendas Del Norte                                 El Paso                  BRO        DW       0.08     1983    Ground    1500               TDS              evaporation pond
Sportsmans World                                    Strawn                   BRO        DW      0.144     1982    Surface   2500             Cl, TDS                surface
Big Bend Motor Inn                                  Terlingua                BRO        DW       0.05     1989    Ground    2900    Ca, Mg, Na, SO4, TDS        evaporation pond
River Oaks Ranch                                    Dallas                   BRO        DW      0.076     1989    Ground    1500            SO4, TDS                surface
Chemical Waste Management                           Port Arthur              BRO        DW      0.066     1989     Other              Hardness, Na, TDS               land
Los Ybanez                                          Los Ybanez               BRO        DW      0.022     1991    Ground                      F, NO3            evaporation pond
Bayside                                             Bayside                  BRO        DW      0.025     1992    Ground                  Fe, Na, TDS               surface
Butterfield Water Systems Inc.                      Pottsboro                BRO        DW      0.04      1992    Ground                    Cl, F, TDS          evaporation pond
Esperanza                                           Esperanza                BRO        DW     0.0576     1994    Ground    1100               TDS              evaporation pond
City of Robinson WTP                                Robinson                 BRO        DW         2      1994    Surface    600               TDS                  surface
City of Kenedy WTP                                  Kenedy                   BRO        DW       0.72     1995    Ground    1300         Cl, TDS, other             surface
Ft. Stockton                                        City of Ft. Stockton     BRO        DW         3      1997    Ground    1400             Cl, Tads               surface
Harlingen Waterworks System                         Harlingen                BRO        WW         4      1999     Other    1200       water reclamation            surface
City of Seymour - RO Plant                          Seymour                  BRO        DW         3      2000    Ground     772        Hardness, NO3               surface
Valley MUD #2 R/O Plant                             Rancho Viejo             BRO        DW       0.25     2000    Ground    2700   TDS, emergency, demand           surface
Granbury                                            City of Granbury         EDR        DW      0.62      1984    Surface   1800                Na                  surface
Oak Trail Shores                                    Dallas                   EDR        DW      0.144     1985    Surface                      TDS                  surface
Lake Granbury                                       Granbury                 EDR        DW        7.5     1989    Surface   1200         Cl, SO4, TDS               surface
Sherman                                             City of Sherman          EDR        DW         6      1993    Surface   1200             THMFP                   sewer
Dell City                                           Dell City                EDR        DW       0.1      1996    Ground    1450             Ca, SO4                  land
San Patricio Municipal Water District Plant C       Ingleside                 MF        DW        7.8     2000    Surface    301    turbidity, water-demand         surface
Travis County Water District #17 WTP                Travis County             MF        DW         2      2002                              biologicals             recycle
Village of Briarcliff                               Briarcliff                MF        DW       0.36     2002
City of San Marcos                                  San Marcos                MF        DW         1      2002
City of Abilene                                     Abilene                   MF        DW         8      2002
Bexar Met. Devel. Corp. Water Production Facility   Von Ormy                  UF        DW         9      1999   Surface    350           biologicals               recycle
Georgetown Utility System South Side WTP            Georgetown                UF        DW         3      2000                                                       sewer
City of Del Rio WTP                                 Del Rio                   UF        DW        16      2002




The sizes of the 20 desalination plants are:
               8 plants of size less than 0.1 MGD or less
               4 plants of size 0.1 to 0.5 MGD
               2 plants of size 0.5 to 1 MGD
               6 plans of size 2 to 7.5 MGD (sizes are 2, 3, 3, 4, 6, and 7.5 MGD)
The 20 desalination plants dispose of the concentrate as follows:
               12 to surface water                                                               plant sizes range from 0.025 to 7.8 MGD
               5 to evaporation pond                                                             plant sizes range from 0.022 to 0.08 MGD
               2 to land                                                                         plant sizes range from 0.066 to 0.1 MGD
               1 to sewer                                                                        plant size is 6 MGD
These trends are similar to national trends with disposal to evaporation ponds and land being
used only by very small plants.

Concentrate Management Challenges
Management of the concentrate produced by desalination processes has become an increasingly
difficult challenge due to several factors that include:
       •       Growing size of plants which limits disposal options
       •       Increased number of plants in a region such that the cumulative effect on receiving waters
               is becoming a limiting factor


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   •   Increased regulation of discharges that makes disposal more difficult and slows the
       permitting process
   •   Increased public concern with environmental issues that plays a role in the permitting
       process
   •   Increased siting of desalination plants in semi-arid regions where conventional disposal
       options are limited
As a result of these trends, it is becoming more and more challenging to find a technically,
environmentally, and financially viable method of dealing with the concentrate.
The concentrate management challenge is particularly acute in the arid southwest US where
frequently disposal to surface water and sewer are not viable options for plants above a small
size. As previously mentioned, approximately 72 % of the plants discharge to surface water or
to sewer. Another 13 percent discharge to deep well; however nearly all of these plants are in
Florida. Thus with the lack of surface water and the growing size of concentrate discharges, the
disposal options used by almost 85% of existing plants are not generally available in the arid
southwest. While the arid climates may support evaporation ponds evaporation ponds are very
expensive except for small volumes. This is reflected in Figure 5, which shows the percentage
use of different disposal options with plant size and in the Texas plant statistics.
Another example of increasing disposal challenges involves limiting the continued degradation
of waterways caused by discharge of higher salinity effluents. In the Denver, Colorado area,
discharge to the South Platte River now requires upstream and downstream modeling / study of
the river including the effects of all other dischargers to determine the water quality-based
feasibility of discharge. A new discharge may impact discharge limits for existing dischargers
and it is a matter of time before new discharges will be severely limited if not prohibited.
The general concentrate disposal situation may be characterized as:
   •   one of urgent challenges in the arid southwest where there are few, if any, feasible
       disposal options for large plants
   • one of a transition stage in the rest of the U.S. where 10 years ago there were few
       disposal challenges and 10 year from now there will be many disposal challenges.
This then is the context of concentrate management considerations.

Concentrate Disposal Options and Cost Factors
Cost considerations
While membrane production costs have been decreasing due to less expensive membranes,
longer membrane life, energy recovery improvements, etc., the cost of concentrate management
has increased. Thus the cost of concentrate management is becoming an increasing percentage
of the total plant cost.
Developing costs for concentrate disposal options is somewhat different than developing costs
for a treatment process. For most concentrate management options:
   •   conveyance costs are site-dependent,
   •   there is less standardization of design (due to more design variables or more design
       decisions that require making), and


                                               179
   •   frequently multiple processing / handling steps may be required.
For the purpose of discussing relative costs, it is assumed that several conditions have been or
could be met. These include:
   •   Each management option is available
   •   Each management option can be permitted
Also for the purpose of leveling the costs, it is assumed that the costs of conveyance of the
concentrate to the site of disposal are the same for each disposal option. This leaves the cost of
the particular disposal option as the only issue of question.
Each option is discussed below as to the design factors and cost parameters.

Disposal options and cost factors
The following information is substantially drawn from two papers submitted for publication /
presentation (AWWA Membrane Residuals Management Subcommittee, 2004 and Mickley,
2004b)
Surface water discharge. Discharge of desalination concentrate to a surface water body (river,
lake, lagoon, canal, ocean, etc.) is the most common management practice, primarily because
this method frequently has the lowest cost and most plants are located relatively near surface
water. Costs for disposal are typically low provided that pipeline conveyance distances are not
excessively long and the concentrate is compatible with the environment of the receiving water
body.
The primary environmental concern is compatibility of the concentrate with the receiving water.
An assessment of salinity or TDS impact as well as those of specific constituents on the
receiving stream is undertaken. Rarely can a higher salinity concentrate be discharged into
lower salinity water if the resulting salinity is more that 10% higher than the upstream receiving
waters. Some facilities address this by dilution of the concentrate with other water such as other
surface water or groundwater, WWTP effluent, cooling water, etc.
Dissolved gases and lack of oxygen can also be concerns for concentrate disposal. Concentrates
from the treatment of most groundwater have very low levels of dissolved oxygen (DO). Prior to
discharge, DO levels must be increased to avoid negative impacts on receiving stream biota. If
the groundwater contains hydrogen sulfide, hydrogen sulfide in the concentrate must be suitably
reduced before its discharge to prevent negative effects. ED concentrate typically contains free
chlorine, which must be neutralized using a reducing agent such as sodium bisulfite compatible
with the receiving stream. As reflected in Figure 7, discharge to surface waters has been used
with all sized concentrates.
Surface Water Discharge Costs. The costs for surface water discharge are influenced by a great
number of site specific factors and are difficult to generalize. The key factors that determine the
costs of concentrate discharge to surface water are:
   •   Conveyance costs to transport the concentrate from the desalination membrane plant to
       the surface water discharge outfall;
   •   Costs for outfall construction and operation;
   •   Costs associated with the monitoring of the environmental effects of the concentrate
       discharge on the surface waters.


                                                180
The costs for the concentrate conveyance are typically closely related to the concentrate volume
and the distance between the desalination membrane plant and the discharge outfall. The outfall
construction costs are very site specific. In addition to the outfall size and diffuser system
configuration, which is driven by the concentrate volume and salinity, these costs are dependent
on the outfall length and material, which in turn depends on the site specific surface water body
hydrodynamics conditions. The outfall discharge operating costs are closely related to the need
to aerate the concentrate before its disposal or to otherwise treat it if it exhibits whole effluent
toxicity. These costs also very widely depending on if existing outfall is used or a new outfall
has to be constructed. The costs associated with environmental monitoring in the case of surface
water discharge may be significant, especially if the discharge is in the vicinity of an impaired
water body, environmentally sensitive area or area of limited natural flushing.
Sewer. Sanitary sewer discharge of a small volume of concentrate usually represents a low cost
disposal method with limited permitting requirements. The adequacy of sewer capacity and
wastewater treatment plant capacity must be addressed. In addition, wastewater effluent quality
will change but must still comply with the wastewater treatment plant’s discharge permit. If the
concentrate salinity and flow levels are significant, impacts of salinity on the biological
efficiency of the wastewater plant should be considered. These capacity and/or quality criteria
may limit the amount of NF, RO or ED concentrate discharged to the sewer. As shown in Figure
7, discharge to sewer is used more often with smaller and medium sized plants than larger plants
due to the effects of larger volume concentrate on the WWTP system.
The WWTP may charge a discharge fee. These are sometimes low, however, the portion of the
wastewater treatment plant capacity utilized by the discharge may be considered as a disposal
cost. In some situations a one-time ‘buy-in’ cost has been charged based on this consideration.
Sanitary Sewer Discharge Costs. Sanitary sewer discharge conditions are usually very site-
specific and the key cost elements for this disposal method are the cost of conveyance (pump
station and pipeline) and fees for connecting to the sanitary sewer, and for treatment/disposal of
the concentrate at the wastewater treatment plant. While the conveyance costs are mainly driven
by the volume of the concentrate, the sewer connection and treatment fees can vary significantly
for a given location from none to several orders of magnitude larger than the conveyance costs.
The sewer connection fees usually are related to the available capacity of the sewer facilities and
the effect of the concentrate discharge on the operational costs of the wastewater treatment plant,
which would provide ultimate treatment and disposal of the concentrate. The connection fees
typically depend on the wastewater utility's willingness to take on the volume and the waste
stream discharge characteristics such as TDS and heavy metal loads. These fees can be quite
large and prohibitive.
Land application. Land application can provide a beneficial reuse of water when membrane
concentrates are applied to vegetation, such as irrigation of lawns, parks, or golf courses. Factors
associated with land application include the water quality tolerance of target vegetation to
salinity, the ability to meet ground water quality standards, the availability and cost of land,
percolation rates, and irrigation needs. An assessment of the compatibility with target vegetation
is conducted, including assessment of the sodium adsorption ratio (SAR), trace metals uptake,
and other vegetative and percolation factors. Regulations governing ground water quality and
protection of drinking water aquifers are investigated to confirm the acceptability of this
alternative. Usually dilution of the concentrate is required to meet groundwater standards.
Where salinity levels are excessive, special salt tolerant species (halophytes) could be considered


                                                 181
for irrigation. Land application also includes the use of percolation ponds and rapid infiltration
basins. In general, land application is used only for smaller volumes of concentrates. These
options are frequently limited by availability of land and/or dilution water. They may also be
limited by climate in locations where land application is not possible year around.
Spray Irrigation Costs. Spray irrigation is possible only if the concentrate meets groundwater
compatibility limits and a level acceptable for crops/vegetation irrigation. Feasibility depends on
the type of the crops/vegetation and on the soil uptake rates. Any blending with a fresh water
source to reduce its salinity may increase cost. The key cost factors of this disposal method are
the costs of land, the storage and distribution system costs, costs of dilution water, and the
irrigation system installation costs, which in turn are driven by the concentrate volume and
salinity. As reflected in figure 7, spray irrigation is used for very small systems due to limited
economy of scale.
Deep well injection. Regulatory considerations for deep well injection or other subsurface
injection alternatives include the transmissivity and TDS of the receiving aquifer and the
presence of a structurally isolating and confining layer between the receiving aquifer and any
overlying Underground Source of Drinking Water (USDW). A USDW is considered when any
water bearing formation contains less than 10,000 mg/L TDS. Deep wells are not feasible in
areas subject to earthquakes or where faults are present that can provide a direct hydraulic
connection between the receiving aquifer and an overlying potable aquifer. A tubing and packer
design is commonly required to allow monitoring of well integrity. One or more small-bore
monitoring wells in proximity to the disposal well are also typically required to confirm that
vertical movement of fluid has not occurred.
The capital cost for deep well injection is higher than surface water disposal, sewer disposal, and
land application in cases where these alternative methods do not require long transmission
pipelines. As reflected in Figure 5, disposal to deep wells is usually restricted to larger volume
concentrates where economies of scale make the disposal option more affordable. Geologic
characteristics are not appropriate for deep well injection in many areas of the United States. A
backup means of disposal must be available for use during periodic maintenance and testing of
the well.
Deep Well Injection Costs. The key factors that influence deep well injection costs are the well
depth and the diameter of well tubing and casing rings. Several other key cost factors are (1) the
need for concentrate pretreatment prior to disposal; (2) pump size and pressure which vary
depending on the geologic conditions and depth of the injection zone; (3) environmental
monitoring well system size and configuration; and (4) site preparation, mobilization and
demobilization. Disposal via deep well injection is expensive but does have an economy of scale
that makes it more feasible for larger capacity desalination plants.
Evaporation pond. Solar evaporation is a viable alternative in relatively warm, dry climates with
high evaporation rates, level terrain, and low land costs. Regulations typically require an
impervious lining and monitoring wells, which will increase costs of evaporation ponds. With
little economy of scale, evaporation ponds are usually used only for small volume concentrates.
While evaporation ponds are typically designed to accommodate concentrate for the projected
life of the demineralization facility, precipitation of salts is expected and must be incorporated
into the depth requirements of the pond or provisions must be made for periodic removal and
disposal or beneficial use of precipitated salts. In addition, the ultimate fate of the concentrated


                                                182
`salts and the future regulatory implications should be considered for any evaporation pond
project. Enhanced evaporation systems (Mickley, 2004c) may increase the evaporation rate and
thereby reduce the evaporation area required by a factor of two to six.
Evaporation Pond Costs. The costs of evaporation pond systems are mainly driven by the
evaporation rate (climate); the concentrate volume; the land and earthwork costs; the liner costs
and the salinity of the concentrate, which determines the useful life of the ponds. The main cost
variable is the evaporative area and the largest individual cost is frequently the liner cost –
particularly where double liners are required. Typically, evaporation rates are lower than soil
uptake rates and therefore, disposal of the same volume of concentrate using evaporation ponds
requires more land than disposal by spray irrigation. As reflected in Figure 7, construction costs
for evaporation ponds have little economy of scale and typically become excessive for all but the
smallest plants. The largest municipal plant discharging to evaporation ponds has a capacity of
1.5 MGD and all the others have capacities of less than 0.4 MGD.
Zero Liquid Discharge. Zero liquid discharge systems such as thermal evaporators, crystallizers
and spray dryers are available to reduce concentrate to a solid product for landfill disposal.
However, the cost for these thermal systems is typically much higher than the cost for the
desalination membrane facility, both from a capital and operating (energy) perspective, making
this disposal option infeasible except for very small concentrate flows. In certain situations
instead of processing concentrate to solids (via a crystallizer or spray dryer) the highly
concentrated brine from the brine concentrator may be sent to evaporation ponds. This typically
results in a lower cost option than processing concentrate to solids. Use of high recovery RO
systems in front of the thermal evaporators can reduce costs for waters of limited hardness. The
selective and sequential removal of salts followed by their use may offer promise to reduce zero
liquid discharge costs (Mickley, 2004c). Reducing the cost of zero liquid discharge systems is
one of the major goals of the National Desalination Roadmap (U.S Bureau of Reclamation and
Sandia National Laboratories. 2003).
Zero Liquid Discharge Costs. Achieving zero liquid discharge with brine concentrators or other
methods is usually the least cost effective concentrate disposal method, because it requires the
use of costly mechanical equipment for evaporation, crystallization and concentration
(dewatering) of the salts in the concentrate. Energy costs associated with the evaporation
processing are significant. Although this method has found practical application in industrial
water reuse facilities, according to the year 2002 survey of the US Bureau of Reclamation
(Mickley, 2004a) it has not yet been used for disposal of concentrate from a RO or NF plant.
Others. Other concentrate management and disposal alternatives such as blending with WWTP
effluent or blending with power plant cooling water may facilitate concentrate dilution and
disposal and may be used in combination with disposal methods previously mentioned.
Permitting requirements for blending of concentrate with treated wastewater effluent are
dependent upon the fate of the combined stream. Blending concentrate with cooling water from
power plants using seawater for once through cooling will reduce concentrations of the discharge
and facilitate permitting. Compliance with standard surface water discharge regulations must
still be satisfied.
Use of concentrate for dust suppression, roadbed stabilization, soil remediation, etc. has been
used occasionally for small volume concentrates. This use (reuse) of concentrate will decrease
where environmental testing is required. Concentrate is site-specific in nature and cannot


                                               183
receive a blanket or general approval for such an application. Each plant’s concentrate will need
to be tested. These applications are further limited by the large amount of road / roadbed
surface required for a given amount of concentrate. In addition, due to the cost of transporting
water, reuse options need be relatively local. Use of concentrate in wetlands is receiving some
research attention as is the evaluation of using desalting membrane concentrates as a feed stock
for sodium hypochlorite generation and for solar energy ponds to recovery energy by heat
generation.
Figure 8 depicts the relative capital costs of the different concentrate management options and
reflects economy of scale factors as well as general (relative) level of cost. More detailed design
and cost guidelines are presented in Mickley, 2004a.




                  Figure 8-Relative Capital Cost of Concentrate Management Options


New Directions – Increasing Recovery and More Efficient
ZLD
A growing issue but one that is still in its infancy is that of moving towards sustainable
technologies. While this is not possible in many situations, it is a desirable and ultimately
necessary direction. Disposal of concentrate to surface water and groundwater typically results
in salt load buildup. In such situations, eventually, the salt load buildup can reach a level that
will limit additional discharges.
In areas of limited water resources there is a growing trend of placing a cost on lost water /
resources such as concentrate. While the concept is not well defined for assigning costs to
concentrate disposed by various options, the author has been asked in several instances to assign
a cost to concentrate ranging from $2,500 to $15,000 per AF, an amount generally corresponding
to the cost of buying water rights. This viewpoint places increased value on considering higher
recovery processing.




                                                184
The consideration of alternative or new concentrate disposal options thus is driven by several
factors:
   •   Growing challenges / difficulty of disposing concentrate
         o growing number and size of membrane plants and resultant concentrate volume
         o Increasing regulatory pressures
         o growing public awareness and concern of environmental issues
   •   Ultimate goal / drive towards sustainable technologies
   •   Increased valuing of ‘lost water’
One direction of consideration is for new uses for or reuse of concentrate. This is a worthy
consideration that may help a limited number of sites. In general, uses are local and with
concentrate having a site-specific nature, any general use or reuse of concentrate needs to take
this variability into account.
Another direction of consideration is that of further treatment of concentrate to facilitate
disposal, use, or reuse. This direction includes reducing the volume of concentrate and in the
extreme leads to zero liquid discharge (ZLD) processes. This area has received some recent
attention (Mickley, 2004c) and a recently initiated project (Mickley, 2004d) is focused on ZLD
and volume minimization.
Increasing recovery reduces concentrate volume and increases its salinity. This does not help for
disposal methods where the concentrate eventually communicates with a receiving water
whether a surface water (via surface water disposal or most cases of disposal to sewer) or
groundwater (via land applications). It typically makes the concentrate less compatible (in terms
of salinity) with the receiving water. Increasing recovery may help other disposal options such
as evaporation ponds (now a smaller volume to evaporate), deep well injection (disposal of a
smaller volume), and zero liquid discharge (smaller volume going to high cost thermal
evaporative systems).
Unless disposal options of evaporation ponds or deep well injection are available there is usually
little gained by minimizing the volume of concentrate unless it is minimized as part of a ZLD
processing scheme.
Conventional zero liquid discharge technologies are very energy intensive, which results in high
annualized costs. These costs can be offset somewhat by increasing membrane system recovery
prior to these thermal evaporative systems.
The various means of increasing membrane system recovery are mostly variants of extensive
pretreatment of the feed to a two-stage membrane system or interstage treatment prior to the
second membrane stage. Such increased treatment has its cost and for situations of high
hardness waters such treatment can result in high chemical costs and high solids disposal cost
which offset the lower energy cost resulting from the smaller brine concentrator size.
As a result of the above and other studies, the Bureau of Reclamation project (Mickley, 2004b)
began looking at the selective removal of individual salts from concentrate. Based on salt
solubility and the ionic makeup of a concentrate, a general sequence of salt precipitation may be
inferred. During the course of investigating the possibility and issues of selective salt recovery
the author became aware of an Australian company, Geo-Processors Pty Limited, which
commercially recovers salts from virtually any effluent including membrane concentrates and
seawater (www.geo-processors.com.au). Subsequent communication with Geo-Processors


                                                185
provided information from which to conduct a preliminary evaluation of their technology and its
applicability to treatment of membrane concentrate. Examples of commercial and pilot projects
provided by Geo-Processors showed a variety of applications with some having a net operating
income due to the sale of salts produced. Recently, Geo-Processors has formed a U.S. based
company, Geo-Processors U.S.A., Inc. One of the stated goals of the Desalination Roadmap
(AWWA Membrane Residuals Management Subcommittee. 2004) is to decrease ZLD costs. The
use of selective salt recovery may provide such lower cost scenarios for various sites.

Summary
The challenges of concentrate management are increasing. It is becoming more and more
difficult to find a technically, economically, and environmentally suitable concentrate disposal
option. Further, concentrate disposal costs are increasing and becoming a greater percentage of
the total plant cost. This paper reviewed the framework of concentrate disposal in the U.S. and
discussed emerging trends, challenges, and new directions. Research, education, new
technologies, and process development will be important in addressing these challenges.

References
AWWA Membrane Residuals Management Subcommittee. 2004. Committee Report: Current
   Perspectives on Residuals Management for Desalting Membranes. Submitted for
   publication to AwwA Journal.
Mickley, M. 2004a. Membrane Concentrate Disposal: Practices and Regulation, Second Edition. U.S.
     Department of the Interior, Bureau of Reclamation, Technical Service Center, Water Treatment
     Engineering and Research Group. (submitted for publication).
Mickley, M. 2004b. Costs of Concentrate Management. Paper for presentation at MEDRC
     International Conference on Desalination Costing – Cyprus, December.
Mickley, M.2004c. Treatment of Concentrate. U.S. Department of the Interior, Bureau of
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Description: Review of Concentrate Management Options